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Please Sit with Your Group

Please Sit with Your Group. Please be sure each member of your team has a copy of Spontaneous Reactions Lecture Notes Spontaneous Reactions Problem Set Standard Reduction Potential Table Today’s reporter is the person who’s birthday is closest to today. Next reading assignment:

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Please Sit with Your Group

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  1. Please Sit with Your Group • Please be sure each member of your team has a copy of • Spontaneous Reactions Lecture Notes • Spontaneous Reactions Problem Set • Standard Reduction Potential Table • Today’s reporter is the person who’s birthday is closest to today. • Next reading assignment: • Zumdahl Chapter 11.4

  2. Spontaneous Reactions Edward A. Mottel Integrated, First-Year Curriculum in Science, Engineering and Mathematics

  3. Spontaneous Reaction • A reaction that occurs by itself. • Not "driven" or forced to proceed by some outside energy source at a given temperature. • Thermodynamic concept • Not a kinetic concept

  4. Spontaneous Reaction • Thermodynamic concept • Corresponds to the equilibrium process of converting reactants into products. • Nature moves towards equilibrium "spontaneously". • Not a kinetic concept. • No mention is made about how long it takes. • Spontaneous does not mean immediate or instantaneous.

  5. Spontaneous Reaction • No direct relationship to exothermicity • Many common examples of spontaneous reactions are exothermic, but • spontaneous reactions can also be endothermic. • Spontaneity does depend on temperature.

  6. Ba(OH)2· 8 H2O(s) + 2 NH4SCN(s) Ba(SCN)2(aq) + 2 NH3(g) + 10 H2O(l) Which of the Following Reactions Are Spontaneous? The neutralization of hydrochloric acid by potassium hydroxide Melting of an ice cube at -10 °C Melting of an ice cube at 20 °C

  7. HCl(aq) + KOH(aq) KCl(aq)+ H2O(l) Ba(OH)2· 8 H2O(s) + 2 NH4SCN(s) Ba(SCN)2(aq)+ 2 NH3(g) + 10 H2O(l) H2O(s) H2O(l) (@ -10 °C) H2O(s) H2O(l) (@ 20 °C) Spontaneous Reactions areFavorable Reactions KCl(aq)+ H2O(l) Ba(SCN)2(aq)+ 2 NH3(g) + 10 H2O(l) H2O(s) H2O(l)

  8. Is the Reaction of Aluminum Metal and Copper(II) Ion Spontaneous? A spontaneous reaction has a positive electrochemical cell potential (Ecell > 0). Al(s) + Cu2+(aq) To predict the spontaneity of a reaction, the cell potential (Ecell) must be determined.

  9. Procedure to Determine the Standard Cell Potential • Identify the reactants and products undergoing oxidation and reduction. • Balance each half-cell reaction. • Add together the oxidation and reduction half-cell equations to give the target equation. • Determine the cell potential.

  10. Procedure to Determine the Standard Cell Potential • Determine the cell potential. • Each half-cell has a standard half-cell potential. • Obtained from a Standard Reduction Potential table. • The sum of the half-cell potentials is the standard cell potential.

  11. 3 ´ ( ) 2 ´ ( ) 2 3 2 3 Aluminum Metal Reacts With Copper(II) Ion Break the equation into reduction and oxidation half-cells Cu2+(aq) + 2 e– Cu(s) Al(s) Al3+(aq) + 3 e– Al(s) + Cu2+(aq) Al3+(aq) + Cu(s) Identify the reactants and products of the target equation Multiply each half-cell equation so that the electrons cancel

  12. 3 ´ ( Cu2+(aq) + 2 e– Cu(s) ) 2 ´ ( Al(s) Al3+(aq) + 3 e– ) 2 2 Al(s) + 3 Cu2+(aq) 3 2 Al3+(aq) + 3 Cu(s) 2 3 Does Copper(II) Ion React with Aluminum Spontaneously? E°½= 0.340 V E°½= 1.662 V E°cell= 2.002 V Determine the half-cell potential for each half-cell equation The sum of the two half-cell potentials is the cell potential

  13. Notes About This ReactionCopper(II) Ion Reacts With Aluminium • If the cell potential is positive (Ecell > 0), then the reaction is spontaneous. • If the standard cell potential is positive (E°cell > 0), then the reaction is spontaneous under standard conditions. • E°cell = 2.002 V

  14. all solutions at 1 M concentrations all gases at 1 atm pressure Two Kinds of Cell Potentials • Cell Potential, Ecell • Standard Cell Potential, E°cell • E°cell = Ecell under standard conditions

  15. 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) The reaction of aluminum and copper(II) ion is spontaneous under standard conditions. aluminum ion copper(II) ion Aluminum metal is put into a solution containing 1 M copper(II) ion and 1 M aluminum ion.

  16. Notes about this reaction 6 e– + 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) + 6 e– This is a six electron process (n=6), corresponding to the number of electrons transferred (cancelled).

  17. 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) [Al3+]2 Q = [Cu2+]3 Notes about this reaction The mass-action expression for this process is

  18. Cell Potentials • The magnitude of the cell potential can aid in identifying the most probable product. • The larger the cell potential, the more favorable the reaction. When aluminum metal reacts with copper(II) ion, what oxidation state is copper is produced? zero (copper metal) aqueous copper(I) ion

  19. 3´ ( Cu2+(aq) +e– Cu+(aq) ) 1´ ( Al(s) Al3+(aq) + 3 e– ) Al(s) + 3 Cu2+(aq) 3 3 Determine the Standard Cell Potential E°½= 0.158 V E°½= 1.662 V Al3+(aq) + 3 Cu+(aq) E°cell= 1.820 V

  20. 3 e– + Al(s) + 3 Cu2+(aq) Al3+(aq) + 3 Cu+(aq) + 3 e– E°cell= 1.820 V Questions about this system Is the reaction spontaneous or non-spontaneous under standard conditions? spontaneous

  21. 3 e– + Al(s) + 3 Cu2+(aq) Al3+(aq) + 3 Cu+(aq) + 3 e– E°cell= 1.820 V 6 e– + 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) + 6 e– E°cell= 2.002 V Questions about this system Does copper metal or copper(I) ion preferentially form? copper metal

  22. 1.820 V 2.002 V Does copper metal or copper(I) ionpreferentially form? Al(s) + Cu2+(aq) Cu+(aq) + Al3+(aq) Cu(s) + Al3+(aq)

  23. 3 e– + Al(s) + 3 Cu2+(aq) Al3+(aq) + 3 Cu+(aq) + 3 e– E°cell= 1.820 V Questions about this system How many electrons are transferred? 3

  24. 3 e– + Al(s) + 3 Cu2+(aq) Al3+(aq) + 3 Cu+(aq) + 3 e– E°cell= 1.820 V [Al3+] [Cu+]3 Q = [Cu2+]3 Questions about this system What is the mass-action expression for this process?

  25. 3 e– + Al(s) + 3 Cu2+(aq) Al3+(aq) + 3 Cu+(aq) + 3 e– E°cell= 1.820 V Questions about this system Aluminum metal is placed in a solution of 1 M copper(II) ion and 1 M copper(I) ion and 1 M aluminum ion at 25 °C. If this reaction actually occurred, under what experimental conditions would a cell potential of 1.820 V be obtained?

  26. 2 Cu+(aq) Cu2+(aq) + Cu(s) Help! Could copper(I) ion undergo disproportionation spontaneously? Disproportionation is the process in which a compound undergoes auto-oxidation and reduction. Break this reaction into an oxidation half-cell and a reduction half-cell. If the cell potential is positive, the reaction is spontaneous.

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